If she no longer believes in FC, would she be interested in doing family stockings? So everyone gets something small (or a few small bits) to put into a stocking for the others. You may want to get fruit/sweets for all. And all 3 of you have stockings to open on Christmas morning. Or even, knowing that no FC and no big FC presents, a stocking from you as parents for the morning could still be nice. There are so many nice but small things that can go into a stocking for growing up girls, and even adults, that it doesn't have to be a "childish" thing, indeed, it can be very practical for lots of the little things a girl needs (hair bobbins, nice deodorant or a perfume sample, cotton pads and face cleaning stuff, nail file, nice pens/pencils for school....) - but something small among more grown up things that is a nice little toy and throwback to childhood would be sweet.
She may also like the idea of getting involved in helping others. So doing a shoebox type appeal, or a local giving tree. Or just thinking of a family locally who you think would be under pressure and doing something for them anonymously.
Get her to start getting involved in the preparations. So she might wrap her own presents (ones from her I mean) or help you with ideas for different people. She might be interested in making certain foods (Christmas cookies, cheese twists, homemade lemonade etc). She is old enough to start getting involved in the housework and kitchen chores - so as there is extra cleaning done, give her a job or two.
Think about outings that she'd enjoy. OK, Santa trips are gone. But would she like to go to a Carol service or other musical event? Especially one that is an evening event? Does she still want to go to a Panto? Would she like to go to the movies with her friends one night instead? Are there other Christmas events that are magical but not totally "small child" focussed that she'd like to see locally or within reasonable reach?
Definitely try to have a shopping trip together. Special mother/daughter time. This is not for you to be buying lots, maybe picking up a couple of items, but more to go together, she can do her buying and get advice from you, but also you have a time out getting coffee/cakes or lunch together and a chance to chat. And enjoy watching the people and hearing the bustle and seeing the festive magic on the main streets without being under pressure. Do another evening (probably before the outing) online shopping together to get ideas and buy ahead so that you have less to HAVE to do on the trip, and can enjoy the spirit without the "I'm hot, have 20 bags, it's too crowded and shops are closing" panic - you can say "we've just bought 1 item but here's a nice place to stop and relax, and then head home because it is mad here today, but look at the lovely school choir over there".
We do a Christmas Hamper in our house. After dinner on Christmas Eve, we light a Christmas Candle (youngest in the house lights it) which gets placed in the window to show weary travelers that we have "room at the inn". (Or on the mantelpiece in our case). We have a little service around that - which means that we have a few minutes quiet time, we talk about the good things that happened in the year since last Christmas, and any bad things too. We remember family and friends no longer with us. We think of anyone who is ill. Then we say a decade of the rosary. (We are Catholic, although not particularly religious - but this ceremony has special meaning for us as a small family group).
After that, we open up the box - there are 3 new pairs of PJs for DH, DD and I. My old Christmas slippersocks, I usually get a new pair for DD (DH wouldn't be a wearer of those). A Christmassy bath bomb each for DD and I from Lush. A NAICE hot choc (the lump of choc on a wooden spoon) each. DD's Snowman hot water bottle cover. Last year, I found snowman shaped marshmallows in Ikea for the hot choc. And DD's stocking to hang. While the stocking may no longer be needed, it is nice to have a lovely Christmassy bath and relax before getting into new, fresh pJs for bed. (We're still at the stage of needing DD to get to bed at a reasonable time, at 12 she may be staying up later).